Align the interface of TestHarness defined in gpu_tests/webgl_conformance.py with that in third_party/webgl_conformance.

This is a re-submit of https://codereview.chromium.org/58833003, which was
reverted due to bot failures.

BUG=314997
TBR=kbr@chromium.org

Review URL: https://codereview.chromium.org/63243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233384 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233384